There is an increasing demand for telecommunication capacity as a result of increased Internet traffic, a growing number of telephone lines for telephones, fax, and computer modems, and an increase in other telecommunication services. The enormous capacity of optical networks and communication systems is one means of addressing this increasing demand. Photonic devices for optical network management and wavelength multiplexing and demultiplexing applications have been extensively researched for a number of years.
A significant class of such devices is commonly called “planar light-wave circuits” or “planar light-wave chips” or just PLCs. PLCs comprise technologies wherein complex optical components and networks are disposed monolithically within a stack or stacks of optical thin films supported by a common mechanical substrate such as a semiconductor or glass wafer. PLCs are typically designed to provide specific transport or routing functions for use within fiber-optic communications networks. These networks are distributed over a multitude of geographically dispersed terminations and commonly include transport between terminations via single-mode optical fiber. For a device in such a network to provide transparent management of the optical signals it must maintain the single-mode nature of the optical signal. As such, the PLCs are commonly, though not strictly, based on configurations of single-mode waveguides. Since optical signals do not require return paths, these waveguide configurations do not typically conform to the classic definition of “circuits”, but due to their physical and functional resemblance to electronic circuits, the waveguide systems are also often referred to as circuits.
The standard family of materials for PLCs, widely demonstrated to have superior loss characteristics, is based on silicon dioxide (SiO2), commonly called silica. The silica stack includes layers that may be pure silica as well as layers that may be doped with other elements such as Boron, Phosphorous, Germanium, or other elements or materials. The doping permits control of index-of-refraction and other necessary physical properties of the layers. Silica, including doped silica, as well as a few less commonly used oxides of other elements, are commonly also referred to collectively as “oxides.” Furthermore, although technically the term “glass” refers to a state of matter that can be achieved by a broad spectrum of materials, it is common for “glass” to be taken to mean a clear, non crystalline material, typically SiO2 based. It is therefore also common to hear of oxide waveguides being referred to as “glass” waveguides. Subsequently, the moniker “silica” is used to refer to those silicon oxide materials suitable for making waveguides or other integrated photonic devices. It is important to note that in the context of this invention, other waveguide materials, such as lithium niobate, spin-on glasses, silicon, siliconoxynitride, or polymers, are also appropriate.
In a typical example of a PLC, a waveguide may comprise three layers of silica glass are used with the core layer lying between the top cladding layer and the bottom cladding layer. In some instances, a top cladding may not be used. Waveguides are often formed by at least partially removing (typically with an etching process) core material beyond the transverse limits of the channel waveguide and replacing it with at least one layer of side cladding material that has an index of refraction that is lower than that of the core material. The side cladding material is usually the same material as the top cladding material. In this example, each layer is doped in a manner such that the core layer has a higher index of refraction than either the top cladding or bottom cladding. When layers of silica glass are used for the optical layers, the layers are typically situated on a silicon wafer. As a second example, waveguides comprise three or more layers of InGaAsP. In this example, adjacent layers have compositions with different percentages of the constituent elements In, P, Ga, and As. As a third example, one or more of the optical layers of the waveguide may comprise an optically transparent polymer. Another example of a waveguide comprises a layer with a graded index such that the region of highest index of refraction is bounded by regions of lower indices of refraction. A doped-silica waveguide is usually preferred because it has a number of attractive properties including low cost, low loss, low birefringence, stability, and compatibility for coupling to fiber.
The use of PLCs in optical networks and communications presents challenges inherent to the PLCs themselves. One such challenge is obtaining dimensional control over the waveguides in the PLCs. Variation or fluctuations in the dimensions of the waveguide often deteriorates the performance characteristics of the PLC. The deterioration in performance of a PLC may eventually reduce the capacity or effectiveness of the overall optical system.
The PLCs referred to herein may be formed using standard techniques used in the semiconductor industry to deposit and pattern optical waveguide materials, e.g., (wet-etch, flame hydrolysis deposition (FHD), chemical vapor deposition (CVD), reactive ion etching (RIE), physically enhanced CVD (PECVD), etc.) FIGS. 1A–1D conceptually illustrates one example of a process of fabrication of an optical waveguide. For simplicity of illustration, the waveguide is shown to have a simple geometry. However, it is understood that a waveguide may have a more complex layout/geometry as described below.
FIG. 1A illustrates a substrate 16 with a lower cladding 14 located on the substrate 16 and a core material 12 deposited on the lower cladding 14. Typically the core material 12 has an index of refraction larger than the cladding material. FIG. 1B illustrates a mask 18 which is deposited on the core material 12. The mask 18 may be a metal (hard-mask) or photoresist mask as required by the particular application. In any case, the pattern of the mask 18 is the same as the desired pattern of the waveguide desired. FIG. 1C illustrates the transfer of pattern to form a waveguide 10. As discussed above, the transfer occurs through the use of various etching techniques in which the core material 12 and the mask 18 is removed via the etching process. As illustrated in FIG. 1D, the waveguide 10 is then covered by a top cladding layer 20, which may have the same index as the lower cladding layer 14.
To produce the desired waveguide pattern on a device, there must be a high ratio of the removal rate of the core material to the removal rate of the mask material. The ratio of the removal rate of the core material to the mask material is commonly referred to as “etch selectivity.” However, in situations where there is a low ratio of the surface area of masked material to the surface area of unmasked core material, it is common to experience a low etch selectivity. The low etch selectivity makes it difficult to control the dimensions of the fabricated waveguide. Accordingly, since the total surface area of the waveguide on a PLC is usually 10% of the total surface area of the substrate, it is common to experience a low etch selectivity when etching PLCs.
Prior attempts of addressing the problem with a low etch selectivity, include the use of various hard-mask materials, such as Chrome. However, such attempts posed several considerable disadvantages. For example, etching of a chrome mask requires wet chemicals which do not provides adequate dimensional control of the Critical Dimension CD of the waveguides. Etching and removal of hard-mask materials requires toxic chemicals which demands special handling and waste management. Such demands result in increased production costs. Moreover, the process of depositing the hard-mask leaves deposits inside the etch tool which eventually contaminate the PLC device.
FIGS. 2A–2B demonstrate one example of a problem caused by low etch selectivity. FIG. 2A illustrates a cross section of a waveguide 10 on an ideally formed PLC 22. As shown, in an ideally formed PLC 22 the waveguide 10 will have a width and depth wherein the depth is controlled by the amount of cladding 12 present on the substrate during fabrication. The width, on the other hand, is known as the critical dimension (CD) and should be uniform throughout a profile 26 of the waveguide 10 and throughout the path of the waveguide 10. FIG. 2B illustrates a more common PLC 24 in which the CD is not uniform throughout a profile 28 of the waveguide 10. For illustrative purposes, an ideal profile 26 is illustrated in phantom lines over the actual profile 28 of the waveguide 10. As shown from the illustration, this problem is difficult to overcome as even directional etching techniques, such as reactive ion etching (RIE), have a lateral etching component which affects the profile 28 of the waveguide 10 when etching with a low etch selectivity.
One attempt at reducing variation in core width is taught in U.S. Pat. No. 5,940,555 ('555) entitled OPTICAL MULTIPLEXER/DEMULTIPLEXER, the entirety of which is hereby incorporated by reference. '555 teaches providing artificial waveguides on both sides of an arrayed waveguide diffraction grating in an optical multiplexer/demultiplexer to reduce the maximum variation in core width of the waveguides in the diffraction grating.
However, there remains a need to improve etch selectivity and to improve the dimensional control of the waveguides in the PLC across the entire device substrate: